Exciter lamp operation



m. Q. mum-m HAL 25,4953??? EKCITER LAMP OPERATION Filed Oct. 21, 1946 23 M mm A FIG. I

RICHARD C. CURTIS AND STEPHEN J. MARTIN IN VEN TOR.

BYQWMM ATTORNEY Patented Feb. 7, 1950 UNITED? STATES? PATENT OFFICE 1.

J. Ma'rtinl hoiig islandi City,-N. Y., assignors to Faximile,Inc.,.Netv-ork,: N. Y., a corporation of Delaware" 6:

The present invention concerns electrical'lamp' operation and; inparticular. methods'of; and

is to provide devices for operating. exciter lamps-- from alternatingcurrent sources'and formain-b taining constant light output not onlyover acycleof the alternating current'butoverlong periods of time in thepresence of line voltage variations.

These and other objectsof the "present inven-* tion willbe"apparent-fro'm"-the detailed descrip tion of the invention given inconnection with the various figures of the drawing.

There are a number of applications in the electronics art for constantlight sources. The exciter lamp used in sound film reproduction mustgive a constant light output particularly over short periods of time asfor instance over the cycle of a sixty cycle wave in order that hum andnoise may be absent in the reproduction of the sound. Similarly, infacsimile pick-up scanners it is necessary that the light source used inscanning be constant so that hum may not be present in the picturesignal. In facsimile it is also necessary that the light be constantover considerable periods of time as well as over the cycle of a sixtycycle wave so that the signal level may remain constant and the averagedensity of the reproduced picture will not be affected by variations inthe pick-up scanner light source. In the past it has been necessary touse complicated and expensive methods for operating the exciter lamp inthese various devices. One system, which has been used utilizes storagebatteries for supplying the lamp and is expensive in the cost andmaintenance of the storage batteries and in the charging equ pmentnecessary to keep them in charged condit on. Another method which hasbeen used includes expensive rectifiers and filters for convertingalternating current from a power line into well regulated constantdirect current for the lamp operation. According to the presentinvention alternating current from" a powerdmc' is converted into squarewave alternating current 1 in which theamplitude of each half cycle iscon-1 stant but although it reverses polarity, the transition from onepolarity to the other takes place so rapidly'"that the thermal inertiaof the lamp'" prevents its temperature from changing. "The lamptemperature, and hence the'light output; is not aifected by the polarityof the currentflowing but only by its absolute value which remainsconstant with this square wave" operation. In the drawing:

Fig. I shows one form of the present invention. 1 Fig 2 shows variousvoltage andicurrent curves:

useful in explain ng "the *operations of Fig. 1'.

In'Fig. I is shown" a source of alternatirrgcur-fl. rent l-, connectedacross a condenserZ, and in series to primary of a transformer 3, insuch a Way that primary 4, andcondenser 2 are in series across thesourceof alternating current IL. The secondary'S, of transformer tis connectedacross exciterlamp'fi. Transformer- 3' is so constructed f that-"itwillsaturate at some desired point and will tend to maintain a constantor substantially constant secondary voltage. The point at which Primaryof transformer 3, 448 turns of No. 20

wire.

Secondary of transformer 3, 26 turns of No. 18

wire.

Core of transformer 3, standard radio power transformer steel .018"thick.

Length of fiux path in transformer 3, 7

Stack thickness, 0.8".

Cross section of center leg, 1 sq. inch.

Condenser 2, 16 microfarads.

It was found that the circuit as described above gave an exciter lamp inwhich the light output was considerably more constant than when operatedfrom a sixty cycle alternating current source and was satisfactory forcertain applications in a facsimile pick-up scanner. In addit on theretoit was found that the output current was constant to within about 0.4%when the primary voltage varied on the order of 10%. While the actualwaveform of the secondary current in any particular case will dependupon the circuit constants, curve B of Fig. 2 shows the waveformobtained with the constants given above. This represents a considerableimprovement in attaining hum-free operation over a pure sine wavecurrent operation.

It has been found, furthermore, that if a second transformer l isconnected with a secondary in series with secondary 5 across lamp 6 andits primary 8 across source i, that the sine wave current as shown incurve A of Fig. 2 will add to the current as shown in curve B totransformer 3 and condenser 2. This sine wave from trans former i willbe displaced in phase as shown and will add to the current of curve B insuch a way that a more nearly fiat or square wave output current will beobtained in lamp 6 as shown at C light output from a lamp operated fromasource of alternating current which includes, in combination, a lamphaving substantial thermal inertia, an alternating current transformerhaving at least a primary and a secondary and capable of considerablecurrent saturation, connections between a source of alternating currentand said primar including an electrical capacitor in series to limit theprimary current and connections between said secondary and said lamp toprovide a flattened current wave to said lamp.

2. A circuit for providing substantially constant light output from alamp operated from a source of alternating current which includes incombination, an alternating current transformer including a primary anda secondary, said transformer saturating at a predetermined primaryvoltage to provide a flattened secondary current, a lamp exhibitingsubstantial thermal inertia connected to said secondary, and a capacitorconnected between said primary and a source of alternating current forlimiting current to said primary in the region of said saturation foroperating said lamp at a substantially constant temperature.

3. A circuit for providing substantially constant light output from alamp operated from a source of alternating current which includes, incombination, a-first alternating current transformer including a primaryand a secondary, said transformer saturating at a predetermined primaryvoltage to provide a flattened secondary current, a capacitor connectedbetween said primary and a source of alternating current for limitingcurrent to said primary in the region of said saturation, a secondalternating current transformer including a primary and secondary,connections between the last said primary and said source of alternatingcurrent, a lamp exhibiting substantial thermal inertia, and connectionsfor placing both of said secondarie in series and across said lamp foroperating said lamp at a substantially con-- stant temperature.

RICHARD C. CURTIS. STEPHEN J. MARTIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,930,127 Klinkhamer Oct. 10,1933 2,341,446 Klinkhamer 1. Feb. 8, 1944 FOREIGN PATENTS Number CountryDate 463,947 Great Britain Apr. 5, 1937

